1. Field of the Invention
The present invention relates to an aluminum alloy fin material for heat exchangers. More particularly, the present invention relates to an aluminum alloy fin material for heat exchangers made of an aluminum alloy, which is manufactured by joining a fin and a constituent material for a working fluid passage by brazing, such as a radiator, heater core, oil cooler, intercooler, and condenser and evaporator for a car air-conditioner, in particular, to an aluminum alloy fin material excelling in intergranular corrosion resistance and joinability, and to a heat exchanger including the fin material.
2. Description of Background Art
A heat exchanger made of an aluminum alloy is widely used as an automotive heat exchanger such as a radiator, heater core, oil cooler, intercooler, and evaporator and condenser for a car air-conditioner. The heat exchanger made of an aluminum alloy is assembled by combining an aluminum alloy fin material with an extruded flattened tube (working fluid passage material) made of an Al—Cu alloy, Al—Mn alloy, Al—Mn—Cu alloy, or the like, or with a tube obtained by forming a brazing sheet in which a filler metal is clad on the above alloy in a tubular shape, and brazing the combined product by flux brazing using a chloride flux, inert gas atmosphere brazing using a fluoride flux, or vacuum brazing through a filler metal.
As the filler metal, an Al—Si filler metal is used. The filler metal is disposed on the side of the working fluid passage material, or either one side or both sides of the fin material. The fin material for a heat exchanger made of an aluminum alloy is required to have a sacrificial anode effect in order to protect the working fluid passage material against corrosion, and to have high-temperature buckling resistance (high-temperature sagging resistance) in order to prevent deformation or erosion caused by the filler metal during high-temperature heating for brazing.
In order to satisfy such demands, an Al—Mn alloy such as JIS A3003 and JIS A3203 has been used as the aluminum alloy fin material. Japanese Patent Publication No. 56-12395 proposes providing the sacrificial anode effect to the fin material by making the Al—Mn alloy electrochemically anodic (less noble) by the addition of Zn, Sn, In, and the like. Japanese Patent Publication No. 57-13787 proposes improving sagging resistance by the addition of Cr, Ti, and Zr.
Japanese Patent Application Laid-open No. 2002-155332 proposes making the structure of the aluminum alloy fin material before brazing fibrous in order to improve brazability by improving formability of the fin material into a corrugated fin. This method is effective in improving formability. However, this method has a problem in which the joining rate by brazing is decreased as the crystal grain diameter after brazing is increased, and buckling occurs if the crystal grain diameter is small.
In recent years, reduction of the weight of the automotive heat exchanger has been demanded in order to further reduce the weight of the vehicle. To deal with this demand, reduction of the thickness of the constituent materials for the heat exchanger, such as the fin material and the working fluid passage material (tube material), has progressed. However, in the case of reducing the thickness of the fin material on which the filler metal is clad, since the amount of filler metal flowing toward the braze joint is decreased, a shortage or excessive melting of the filler metal occurs at the joint.
In the heat exchanger made of an aluminum alloy, in order to mainly provide the fin material with an effect as a sacrificial anode material which protects the tube material against corrosion, a material design which allows the fin material to corrode has been taken into consideration. However, a problem occurs in the case of using a constituent material having a reduced thickness, in particular, an aluminum alloy fin material having a thickness of 0.08 mm or less. In the case of using a brazing fin material on which the filler metal is clad, the molten filler metal on both sides of the fin material penetrates into the grain boundaries in the entire area in the direction of the sheet thickness. This causes anodic (less noble) components to be formed at the grain boundaries, whereby intergranular corrosion easily occurs. If intergranular corrosion significantly occurs in the fin material, the strength of the heat exchanger core is decreased. In the case of using a bare fin material on which the filler metal is not clad, the filler metal on the side of the tube combined with the fin material penetrates into the braze joint between the tube and the fin, whereby intergranular corrosion easily occurs. If intergranular corrosion significantly occurs in the fin material, the strength of the heat exchanger core is decreased. Therefore, these fin materials are required to exhibit corrosion resistance while protecting the tube material against corrosion.